Resin-wood composites, such as oriented strand board (“OSB”), wafer board, chipboard, fiberboard, etc., are widely used as construction materials, such as for flooring, sheathing, walls, roofing, concrete forming, and so forth. The wood component typically is virgin or reclaimed ligno-cellulosic material, which may be derived from naturally occurring hard or soft woods, singularly or mixed. Typically, the raw wood starting materials are cut into strands, wafers, chips, particles, or other discrete pieces of desired size and shape. These ligno-cellulosic wood materials can be “green” (e.g., having a moisture content of 5-30% by weight) or dried (e.g., having a moisture content of about 2-18 wt %).
In the commercial fabrication of OSB, for instance, multiple layers of raw wood “flakes” or “strands” are bonded together by a resin binder. The flakes or strands used in OSB production have been made by cutting logs into thin slices with a knife edge parallel to the length of a debarked log. The cut slices are broken into narrow strands generally having lengthwise dimensions which are larger than the widths, where the lengths are typically oriented parallel to the wood grain. The flakes are typically 0.01 to 0.05 inches thick, although thinner and thicker flakes can be used in some applications, and are typically, less than one inch to several inches long and less than one inch to a few inches wide. The raw flakes then may be dried. The raw flakes or other ligno-cellulosic wood materials are coated with a polymeric thermosetting binder resin and a sizing agent such as wax, such that the wax and resin effectively coat the wood materials. Conventionally, the binder, wax and any other additives are applied to the wood materials by various spraying techniques. One such technique is to spray the wax, resin and additives upon the wood strands as the strands are tumbled in a drum blender. Binder resin and various additives applied to the wood materials are referred to herein as a coating, even though the binder and additives may be in the form of small particles, such as atomized particles or solid particles, which may not form a continuous coating upon the wood material. After any drying, the flakes are coated with a thin layer of binder material and sizing agent, and then pressed and resin-cured to provide a desired product shape. This coating operation may be performed in a blender. The coated flakes may then be spread on a conveyor belt to provide a first surface ply or layer having flakes oriented generally in line with the conveyor belt, then one or more plies that will form an interior ply or plies of the finished board is (are) deposited on the first ply such that the one or more plies is (are) oriented generally perpendicular to the conveyor belt. Then, another surface ply having flakes oriented generally in line with the conveyor belt is deposited over the intervening one or more plies having flakes oriented generally perpendicular to the conveyor belt. Plies built-up in this manner have flakes oriented generally perpendicular to a neighboring ply insofar as each surface ply and the adjoining interior ply. The layers of oriented “strands” or “flakes” are finally exposed to heat and pressure to bond the strands and binder together. The resulting product is then cut to size and shipped. Typically, the resin and sizing agent comprise less than 10% by weight of the oriented strand board.
Board product uniformity and quality is sensitive to formulation variations. Often, panel components are not measured directly but inferred from application rates. This situation has led to a gap in information about blending efficiency, which limits the ability to improve the process. There is a need for rapid, noninvasive analysis methods for wood composite products. Direct measurement of the amount of adhesive, wax, moisture, or other binder constituents or additives applied to ligno-cellulosic particles, e.g., OSB flakes, has been a time-consuming procedure. This has been accomplished in the past, for example, by elemental analysis or image analysis. While elemental analysis can give accurate measurements on the elements present in samples, a week or more may be required before results are returned from an outside lab. Delayed acquisition of analysis results may limit their usefulness for near-time adjustment of current process parameters such that considerable production may occur before a formulation variation from target conditions is identified. Elemental analysis is also of limited use for discriminating between and determining the concentrations of components whose elemental makeup contains significant carbon, hydrogen, and/or oxygen, since these are also the elements predominant in wood, and the test results do not differentiate between different sources of these elements. Waxes and polyols are two common OSB components that fall into this category. Other methods of wax analysis are in use, but they involve lengthy organic solvent extraction procedures.
Image analysis also has been used to analyze content of OSB composite wood products. Image analysis involves off-line photographing or scanning individual flakes, or paper onto which resin has been transferred, and using a computer to analyze the digital image. The coverage area of a colored material on a lighter-colored background, such as phenol-formaldehyde resin on a flake, is then calculated. This approach works well for colored components, such as phenol-formaldehyde resin, but not for colorless or light colored components such as isocyanate resins, urea-formaldehyde resins, polyols, or waxes. A dye may be added to the component or sprayed on the treated flake.
Spectroscopic techniques also have been described for monitoring ligno-cellulosic board formulations. All organic materials absorb infrared (including near-infrared) light according to Beer's law. Three categories of infrared spectroscopies are commonly recognized, classified by the energy of the light used, comprising: mid-infrared spectroscopy from 2400-25,000 nm, near-infrared (NIR) spectroscopy from 800-2400 nm, and far-infrared spectroscopy from 20,000-66,000 nm. Far IR is typically used for inorganic materials. Quantitative mid-IR analysis can be problematic due to baseline effects and the absorbance of background gases such as water vapor and carbon dioxide. NIR spectroscopy does not suffer from these difficulties, and it is generally faster and requires less sample preparation than mid-IR. NIR instruments are faster than mid-IR instruments because the energy from the lamp is more intense, the detector is more sensitive, and the Beer's law constant is greater in the NIR region.
NIR spectroscopic analysis of adhesive-treated wood flakes is time-dependent, because the adhesives undergo chemical reactions such as polymerization, even at room temperature. These changes in the chemical makeup of the samples result in changes in their spectra, which can make the spectra unsuitable for component concentration predictions which are related to calibrations based on samples that may have been handled differently after sampling. Conducting rapid spectra acquisition on freshly mixed and collected samples can reduce this variable. However, on-the-spot rapid assay strategies may not be practical for many production operations.
NIR technology has been used in the wood industry, most commonly for moisture measurements. However, it may also be used for resin and wax analysis. U.S. Pat. Nos. 6,846,446 and 6,846,447 describe measuring resin content on engineered wood products by near-infrared (NIR) spectroscopy and a method for calibrating the instruments. The '446 and '447 patents measure resin alone, and remove information about moisture content (and other non-resin components) of the samples before spectral data are analyzed for resin content. Representative industrial NIR spectroscopic instruments, such as one from Process Sensors Corporation (Milford, Mass.), measure only at several different wavelengths, or alternatively at narrow bands of wavelengths, rather than an entire NIR spectrum. While measurements at only a few wavelengths may produce reliable calibrations in limited circumstances, these strategies are relatively inflexible.
There also is a need for noninvasive analysis methods for wood composites that can handle aged samples. Ideally, in order to take spectra of recently prepared resin-loaded samples, an NIR instrument or other spectrophotometer would be located on the same site, on-line or off-line, as the source of the production samples. However, it may not be economical for all operations to purchase an instrument for every production site because of the expense and the need for trained instrument operators. Nor is it feasible to ship a NIR instrument from one location to another because of the risk of damage. Even if production samples and the instrument are located on the same production site, there are often impediments to rapidly collecting spectra from production samples. Particularly in a continuous production mode, multiple tasked personnel may need to be available for the sample collection, sample preparation, and analysis processing to get accomplished regularly and rapidly. Such levels of staffing may not be feasible for all operations during all production runs. Methods for analyzing aged wood composite samples which provide results reflecting the fresh sample formulation are needed. Such methods ideally would make it possible to accurately acquire and collect spectra from wood composite production samples independent of the sample's age, time, and sampling location.
As will become apparent from the descriptions that follow, the invention addresses these needs as well as providing other advantages and benefits.